Crystal seed and method of obtaining it



M E R G L L m E m &

Filed Aug. 209 1947 (FRYSTAL SEED AND METHOD OF OBTAININQ 1'1 FIG. 5

Patented Dec. 21, 1948 f OFFICE CRYSTAL SEED 'AND METHOD OF OBTAINING 1r Bengt a. F. Kjellgren, University assignor to The Heights, Ohio,

Brush Development Company,

Cleveland, Ohio, a corporation of Ohio This invention relates to a method of obtaining it from a large grown mother crystal.

Previous-to this invention it was customary to grow crystals, such as Rochelle salt, from solid, unflawed seeds cut from a mother crystal, as shown in United States Patents Re. 19,697 and Re. 19,698, issued on September 10, 1935, to Bengt Kjellgren and assigned to the assignee of the present invention. It was the practice to use only. clear, solid, unflawed seed material because experience had shown that the quality of the seed material had a direct bearing on the quality of the crystal bars grown therefrom.

A group of crystals of elongated habit and having particularly useful properties are the primary phosphates 'of ammonium and potassium and crystalsisomorphous therewith, which for purposes of convenience we shall refer to as P-type crystals. The term P-type crystal is to be un; derstood as embracing primary ammonium phosphate (NH4H2PO4) primary potassium phosphate, primary rubidium phosphate, the primary arartificially senates of ammonium, potassium and rubidium,

isomorphous mixtures of any of these named compounds, and all other piezoelectrically active crystalline materials isomorphous therewith. In Wyckoif; Structure of Crystal (published by Chemical Catalog Company, 2nd ed., N.-Y., 1931) I this crystal type is introduced as KHzPoi-type.

In the Strukturbericht (supplement to Zietschrift crystal seed and the fuer Kristallographie) this type here called the P-type is designated as type H-2-2.

The habit oi-this P-type crystal is a combination ofthe secondary prism and the secondary bipyramid. It is characteristic for the P-type crystals that they are elongated parallel to the axis of the prism, which isthe optic axis of these crystals and here designated as the Z-axls.

I For most applications of P-type material, such as' in piezoelectric transducers and light modulators, it is expedient to utilize Z-cut plates, the major faces of which lie in planes substantially perpendicular to the optic axis of the mother crystal. This makes it particularly desirable to provide a crystal having a volume of clear material having considerable extension at right angles to the optic axis as well as parallel thereto.

The difficulties which haveto be overcome .in producing P-type crystals containing a large volume of clear material are due to the following factors inherent in the nature of that class of crystals: The natural habit of the P-type crystals is a combination of an elongated tetragonal prism with a tetragonal bi-pyramid, and the rate of Application August 20, 19317, Serial No. 769,576

4 Claims. (01. 171-327) growth in the direction of the optic axis is about ten times as large as the rate of growth at right angles to the direction of the optic axis. Accordingly, most of the material of the crystal is grown by deposition on the pyramid faces and a crystal grown from a'natural seed crystal will be long and of small crosssectional area.

In order to produce crystals having satisfactory extension in the directions of the X and. Y axes and having sufflcient length in the direction of the axis that a number of Z-cut plates can be obtained, the following method has been developed: A Z-cut plate of the crystalline material, having an extension in the direction of the X and Y axes approximately equal to the extension desired in the grown crystal, is planted in a hot saturated aqueous solution of the material to be grown. The temperature of the solution is gradually lowered while the solution is caused to flow the subsequent growth is in the nature of an extension of the prism portion along the Z-axis, and it is this material which, if clear and unflawed, is utilized for transducer and optic plates. Plates cut from this clear material may be used as seed plates for further growth of crystals, in

accordance with the teaching of application Serial No. 728,310, filed in the names of Hans I Jaffe, Edward M. Brazis, and Bengt Kjellgren, for

Method of and means for growing crystals. However, it is wasteful of otherwise usable crystalline material.

An object of the invention is to provide a seed for growing a P-type crystal which does not comprise material suitable fora plate for transducer or optic use.

Another object of the invention is to provide an improved method of producing seed plates of materials of the group described, from which large and substantially uniform crystals may expeditiously be grown.

A further object of the invention is to provide for use as seed plates material which is unfit for use as transducer or optic plates.

Another object of my invention is to utilize substantially all of the material in a'crystal bar either for platesto be sold comlnercially or for seed material.

line material.

materi 'l which heretofore was redissolved in a the process of obtaining a seed plate of P-typecrystalline material from a mother crystal having a flawed center portion by severing a plate from the flawed center portion with the major faces of the plate substantially perpendicular to the Z-axis of the crystalline material.

Other objects and a fuller understanding of my invention may be had by referring to the following description and drawings in which:

Fig. 1 is an isometric view of a seed plate; Fig. 2 is a side view of a seed crystal grown from the seed plate of Fig. 1; Fig. 3 is a side view of a crystal bar grown from the seed crystal of Fig. 2; and Figs. 4, 5, and 6 are views of different kinds of seed plates cut from the crystal bar of Fig. 3 and embodying the invention.

With reference to Fig. 1, there is shown a seed plate ill of crystalline material of the P- type cut as a shear plate: that is with its major faces perpendicular to the Z-axis of the crystal- Such a plate may be used as a piezoelectric transducer as it has been cut from clear uni'lawed crystalline material, or it may be used as a seed plate for growing a seed crystal such as the crystal i2 in Fig. 2. The seed plate ID has two'holes drilled in one edge and mounting pins II are inserted, in those holes. The pins may be of such a size with respect to the holes that a force-fit is established for holding the pins in the plate or a small amount of an adhesive may be used.

The seed plate Illis planted in a tray filled with an aqueous solution of P-type salt and the seed crystal i2 is caused to grow by the method described in detail in my co-pendingapplicaticn Serial Number 681,505, entitled Method of growing P-type seed crystals. After the seed plate ID has grown into a seed crystal I2, it will be noticed that the seed crystal comprises an outside shell IQ of clear, solid, substantially unflawed crystalline material and that the inside i3 is formed of badly flawed material, parts of which may even be in a liquid state. The seed crystal i2 is then planted in a tray filled with an aqueous solution of P-type salt and'caused to grow into a crystal bar i4. After the crystal bar has been grown there is a substantial amount of clear, unflawed crystalline material lying on each side of the seed crystal between the tapered end portion 9 of the seed crystal l2 and the base 8 of the tapered end portion ofthe crystal bar. These two sections may be used for obtaining unflawed transducer material, and to use them for seed plate material is an economic waste.

I have found, contrary to previous concepts. that it isnot necessary to use clear, solid, unflawed crystalline material for growing P-type crystals, and by this discovery I have made available for tranducer or other use all those Z-cut plates which, under the prior practice, were used as seed plates. About eight plates of clear material 2 x 2%" x /1" thick are usually obtained from the average P-type mother crystal, and in the past one of thoseplates was utilized as a seed plate and the other seven were used commercially. Since my discoverey all eight plates are used commercially. Thus, it has increased the production of salable plates by about twelve per cent. The flawed and otherwise unusable plates for seed material are obtained by cutting through the seed crystal portion of the mother crystal M to obtain plates whose major faces are perpendicular to the Z-axis of the crystalline material. The dot-dash lines in Fig. 3 indicate lines along which cuts are made to obtain the seed plates.

Fig. 4 illustrates one such seed plate 16. The centerportion may be entirely hollow but it is important that the frame-like seed plate l5 have clear, solid, substantially unflawedrrystalline material in its peripheral edge portion.

Fig. 5 illustrates another such seed plate I6 cut from the crystal bar H3 at a location near the end 9 of the seed crystal. The clear solid frame is larger than in plate l5 and the hollow center portion is much smaller. A flawed portion I3 is apt to exist between the hollow central portion, but I have found that the existence of the hollow center or of the badly flawed center is substantially immaterial to the growth of well-formed seed crystals of P-type material so long as an outside frame of clear solid material exists. This is due to the fact that the crystalline material which is deposited on the major faces of a seed plate tends to form as clear solid material first on the outside peripheral edge portions.

This clear solid edge portion tapers inwardly as the crystal grows thereby forming the typical P- type crystal shape and enclosing flawed material and mother liquor within the hard outside shell.

At the end of a few days of growth the seed crystal l2 has completely formed and the ends 9 are sealed by clear hard crystalline material.

Sufllcient shell material. should be allowed to deposit that the seed crystal is solid enough to withstand reasonable handling without causing the shell to break, and it should be of suflicient.

thickness that it may be sawed in a direction perpendicular to the Z-axis to obtain an unbroken frame. The seed crystal may then be picked, inspected and replanted to grow 'the crystal bar in accordance with the process claimed in application Serial No. 675,533, for Method of growing crystals, filed by Edward J. Malone, and assigned to the assignee of the present invention.

Another type'of seed plate which may be obtained from the mother crystal I4 is shown in Fig. 6. It is obtained by cutting the original seed plate ill from the center of the grown crystal bar l4, thus obtaining a seed plate which already has the pins II in place. Y

If the cut line'is closely adjacent to but not touching the face of the original seed plate ID, a seed plate I! is obtained which has on three sides a very thin frame of clear, solid, unfiawed material and a rather thick frame on the side where the pins Hare positioned. This is due to the fact that the presence of pins seems to promote accelerated crystalline growth in that direction. The face of the seed plate between the clear solid edges will be formed of a series of ridges running either transversely or vertically across the plate. These ridges constitute flawed material but do not adversely influence the growth of a seed crystal from a seed plate I'I.

Another type of seed 'plate which may be cutfrom the crystal bar i4 is illustrated as plate [8 I in Fig. 3. This plate has its righthand major face formed of clear unflawed material and the lefthand major face has its central portion hollow or formed of flawed material, but these flaws do not extend completely through the plate. Thus, there is provided a major face of unflawed material, similar to a face of the plate [0, upon which tion is only to be limited by the scope of the claims.

I claim as my invention:

1. As an article of manufacture to be used in a solution as a seed to promote the growth of clear substantially flawless P-type crystalline material, a plate of P-type crystalline material having its two major faces substantially perpendicular to the Z-axis of the crystalline material, said plate comprising a frame of clear solid mate rial.

2. The invention set forth in claim 1, further characterized in this, that said frame of clear solid material has an opening within its boundary devoid of crystalline material. v

3. The invention set forth inclaim 1, further characterized in this, that said frame of clear solid material encloses an area of flawed crystalline material.

4. The process of obtaining a seed plate of P-type crystalline material from a mother crystal having a flawed center portion comprising severing a plate from the said flawed center portion, the major faces of said plate being substantially perpendicular to the Z-axis of the crystalline material.

BENGT RIF. KJELLGREN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Stanbel May 1, 1934 Number 

